The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This invention relates to compositions and methods for treating subterranean formations, in particular, compositions and methods for cementing and completing thermal recovery wells.
During the construction of subterranean wells, it is common, during and after drilling, to place a tubular body in the wellbore. The tubular body may comprise drillpipe, casing, liner, coiled tubing or combinations thereof. The purpose of the tubular body is to act as a conduit through which desirable fluids from the well may travel and be collected. The tubular body is normally secured in the well by a cement sheath. The cement sheath provides mechanical support and hydraulic isolation between the zones or layers that the well penetrates. The latter function is important because it prevents hydraulic communication between zones that may result in contamination. For example, the cement sheath blocks fluids from oil or gas zones from entering the water table and polluting drinking water. In addition, to optimize a well's production efficiency, it may be desirable to isolate, for example, a gas-producing zone from an oil-producing zone.
The cement sheath achieves hydraulic isolation because of its low permeability. In addition, intimate bonding between the cement sheath and both the tubular body and borehole is necessary to prevent leaks. However, over time the cement sheath can deteriorate and become permeable. Alternatively, the bonding between the cement sheath and the tubular body or borehole may become compromised. The principal causes of deterioration and debonding include physical stresses associated with tectonic movements, temperature changes and chemical deterioration of the cement.
Development of heavy oil reserves often involves applying heat to the producing reservoir. Such thermal-recovery wells frequently employ steam injection. Steam injection encompasses a number of techniques, including steam assisted gravity drainage (SAGD), cyclic steam stimulation (CSS) and steamflooding. During such operations, the resulting well temperature may vary from 150° to 700° C., subjecting the cement sheath to especially severe stresses and possibly leading to cement-sheath failure, formation of microannuli or both. Indeed, a significant percentage of thermal-recovery wells suffer from various forms of leaks including complete steam breakthrough to surface.
There have been several proposals to solve the problems of cement-sheath deterioration. One approach is to design the cement sheath to mechanically survive physical stresses that may be encountered during its lifetime (U.S. Pat. No. 6,296,057). Another approach is to employ additives that improve the physical properties of the set cement. U.S. Pat. No. 6,458,198 describes the addition amorphous metal fibers to improve the strength and impact resistance. EP 1129047 and WO 00/37387 describe the addition of flexible materials (rubber or polymers) to confer a degree of flexibility to the cement sheath. WO 01/70646 describes cement compositions that are formulated to be less sensitive to temperature fluctuations during the setting process. However, these solutions are not as effective in the context of thermal-recovery wells. The stresses may be too severe for the solutions to be effective, the active material may not be stable at such high temperatures, or both.
Therefore, despite the valuable contributions of the prior art, a need still remains for improved cement formulations that can better withstand the thermal and mechanical stresses associated with thermal-recovery wells.